Controllable-load circuit for use with a load control device

ABSTRACT

A load control device for controlling the amount of power delivered from an AC power source to an electrical load is operable to conduct enough current through a thyristor of a connected dimmer switch to exceed rated latching and holding currents of the thyristor. The load control device comprises a controllable-load circuit operable to conduct a controllable-load current through the thyristor of the dimmer switch. The load control device disables the controllable-load circuit when the phase-control voltage received from the dimmer switch is a reverse phase-control waveform. When the phase-control voltage received from the dimmer switch is a forward phase-control waveform, the load control device is operable to decrease the magnitude of the controllable-load current so as to conduct only enough current as is required in order to exceed rated latching and holding currents of the thyristor.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from commonly-assigned U.S. ProvisionalPatent Application Ser. No. 61/263,204, filed Nov. 20, 2009, entitledCONTROLLABLE-LOAD CIRCUIT FOR USE WITH A LOAD CONTROL DEVICE, the entirecontents of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a load control device for ahigh-efficiency light source, such as a light-emitting diode (LED) lightsource or a compact fluorescent lamp, and more particularly, to acontrollable-load circuit for use in an LED driver for controlling theintensity of an LED light source.

2. Description of the Related Art

Light-emitting diode (LED) light sources are often used in place of oras replacements for conventional incandescent, fluorescent, or halogenlamps, and the like. LED light sources comprise a plurality oflight-emitting diodes mounted on a single structure and provided in asuitable housing. LED light sources are typically more efficient andprovide longer operational lives as compared to incandescent,fluorescent, and halogen lamps. In order to illuminate properly, an LEDdriver control device (i.e., an LED driver) is typically coupled betweenan alternating-current (AC) source and the LED light source forregulating the power supplied to the LED light source. The LED drivermay regulate either the voltage provided to the LED light source to aparticular value, the current supplied to the LED light source to aspecific peak current value, or may regulate both the current andvoltage.

LED drivers may be controlled by standard two-wire dimmer switches. Atypical dimmer switch comprises a bidirectional semiconductor switch,e.g., a thyristor (such as a triac) or two field-effect transistors(FETs) in anti-series connection. The bidirectional semiconductor switchis coupled in series between the AC power source and the LED driver andis controlled to be conductive and non-conductive for portions of ahalf-cycle of the AC power source to thus control the amount of powerdelivered to the LED driver (e.g., using a phase-control dimmingtechnique). A “smart” dimmer switch (i.e., a digital dimmer switch)comprises a microprocessor (or similar controller) for controlling thesemiconductor switch and a power supply for powering the microprocessor.In order to properly charge, the power supply of a two-wire dimmerswitch must conduct a charging current I_(CHRG) from the AC power sourcethrough the LED driver when the bidirectional semiconductor switch ofthe dimmer switch is non-conductive each half-cycle to provide anappropriate amount of voltage to the microprocessor (and any other lowvoltage circuitry).

Generally, dimmer switches use either a forward phase-control dimmingtechnique or a reverse phase-control dimming technique in order tocontrol when the bidirectional semiconductor switch is renderedconductive and non-conductive to thus control the power delivered to theLED driver. With forward phase-control dimming, the bidirectionalsemiconductor switch is turned on at some point within each AC linevoltage half-cycle and remains on until the end of the half cycle.Forward phase-control dimming is often used to control the powerdelivered to a resistive or inductive load, which may include, forexample, a magnetic low-voltage transformer or an incandescent lamp.Since a triac becomes non-conductive when the magnitude of the currentconducted through the triac decreases to approximately zero amps, triacsare typically used for forward phase-control dimming. With reversephase-control dimming, the bidirectional semiconductor switch is turnedon at the zero-crossing of the AC line voltage and turned off at somepoint within each half-cycle of the AC line voltage. Reversephase-control dimming is often used to control the power delivered to acapacitive load, which may include, for example, an electroniclow-voltage transformer. Since the bidirectional semiconductor switchmust be rendered conductive at the beginning of the half-cycle, and mustbe able to be turned off within the half-cycle, reverse phase-controldimming typically requires that the dimmer switch have two FETs inanti-serial connection, or the like.

As previously mentioned, the bidirectional semiconductor switch of thedimmer switch may be implemented as a thyristor, such as a triac or twosilicon-controlled rectifiers (SCRs) coupled in anti-parallelconnection. Thyristors are typically characterized by a rated latchingcurrent and a rated holding current. The current conducted through themain terminals of the thyristor must exceed the latching current for thethyristor to become fully conductive. In addition, the current conductedthrough the main terminals of the thyristor must remain above theholding current for the thyristor to remain in full conduction. However,when a dimmer switch is controlling a high-efficiency electrical load,such as an LED light source, the current conducted through the mainterminals of the thyristor may likely not exceed the rated latchingcurrent or the rated holding current.

Some prior art lighting control systems for LED light engines haveincluded additional load circuits coupled in parallel with the LED lightengines for conducting enough current to ensure that the latchingcurrent and holding current of the thyristor in the dimmer switch areexceeded. In addition, the load circuit may also conduct the chargingcurrent of the power supply of the dimmer switch to ensure that thepower supply is able to stay charged. The load circuit may comprise, forexample, a fixed load, such as a resistor, or a variable load circuitthat can provide two different load characteristics during eachhalf-cycle. An example of a prior art variable load circuit is describedin greater detail in co-pending, commonly-assigned U.S. patentapplication Ser. No. 12/438,587, filed Feb. 24, 2009, entitled VARIABLELOAD CIRCUITS FOR USE WITH LIGHTING CONTROL DEVICES, the entiredisclosure of which is hereby incorporated by reference. The prior artload circuits conduct current through the connected dimmer switches atall times when the load circuits are installed. However, it may not benecessary for the load circuits to conduct the current through thedimmer switch at all times. In other words, the prior art load circuitsmay unnecessarily waste power by conducting current through the dimmerswitch when not required. Therefore, there is a need for acontrollable-load circuit that is able to conduct the appropriate amountof current through the connected dimmer switch only when needed.

SUMMARY OF THE INVENTION

According to a first embodiment of the present invention, a load controldevice for controlling the amount of power delivered from an AC powersource to an electrical load comprises a load control circuit, acontroller, and a controllable-load circuit configured to conduct acontrollable-load current having a constant magnitude that may beadjusted by the controller. The load control device is adapted to becoupled to the AC power source through a dimmer switch, such that thecontrollable-load current is conducted from the AC power source throughthe dimmer switch. The load control circuit is adapted to be coupled tothe electrical load for controlling the amount of power delivered to theelectrical load. The controller is coupled to the load control circuitfor controlling the amount of power delivered to the electrical load inresponse to a conduction period of a phase-control voltage received fromthe dimmer switch. The controller is configured to monitor theconduction period of the phase-control voltage and to cause thecontrollable-load circuit to decrease the magnitude of thecontrollable-load current from an initial magnitude in a firsthalf-cycle to a decreased magnitude in a second subsequent half-cycle,such that the conduction period of the phase-control voltage when thecontrollable-load current has the decreased magnitude in the secondhalf-cycle is the same as when the controllable-load current has theinitial magnitude in the first half-cycle.

According to a second embodiment of the present invention, a loadcontrol system for controlling the amount of power delivered from an ACpower source to an electrical load comprises a load control device and adimmer switch. The load control device is adapted to be coupled to theelectrical load for controlling the amount of power delivered to theelectrical load. The dimmer switch comprises a thyristor coupled inseries electrical connection between the AC power source and the loadcontrol device, and the thyristor is characterized by a rated holdingcurrent and is configured to conduct a load current from the AC powersource to the load control device. The dimmer switch is configured togenerate a phase-control voltage that is provided to the load controldevice to enable the load control device to adjust the amount of powerbeing delivered to the electrical load in response to a conductionperiod of the phase-control voltage. The load control device isconfigured to conduct a controllable-load current from the AC powersource through the thyristor of the dimmer switch. The load controldevice is configured to maintain the magnitude of the controllable-loadcurrent constant for at least a portion of each half-cycle of the ACpower source. The load control device is configured to decrease themagnitude of the controllable-load current from an initial magnitude ina first half-cycle to a decreased magnitude in a second subsequenthalf-cycle, such that the decreased magnitude of the load currentconducted through the thyristor in the second half-cycle exceeds therated holding current.

According to a third embodiment of the present invention, a load controlsystem for controlling the amount of power delivered from an AC powersource to first and second electrical loads, comprises a first loadcontrol device, a second load control device, and a dimmer switch. Thefirst load control device is adapted to be coupled to the firstelectrical load for controlling the amount of power delivered to thefirst electrical load, and the second load control device is adapted tobe coupled to the second electrical load for controlling the amount ofpower delivered to the second electrical load. The dimmer switchcomprises a thyristor coupled in series electrical connection betweenthe AC power source and the first and second load control devices, andthe thyristor characterized by a rated holding current and configured toconduct a load current from the AC power source to the first and secondload control devices. The dimmer switch is configured to generate aphase-control voltage that is provided to the first and second loadcontrol device to enable the first and second load control devices toadjust the amount of power being delivered to the respective electricalloads in response to a conduction period of the phase-control voltage.The load control devices are configured to conduct respectivecontrollable-load currents from the AC power source through thethyristor of the dimmer switch. The load control device is configured tomaintain the magnitude of each controllable-load current constant for atleast a portion of each half-cycle of the AC power source. The loadcontrol devices are each configured to adjust the constant magnitude ofthe respective controllable-load current from an initial magnitude in afirst half-cycle to a decreased magnitude in a second subsequenthalf-cycle, such that a total magnitude of the respectivecontrollable-load currents exceeds the rated holding current when thecontrollable-load currents have the respective decreased magnitudes inthe second half-cycle.

According to a fourth embodiment of the present invention, a loadcontrol device controls the amount of power delivered from an AC powersource to an electrical load and is adapted to be coupled to the ACpower source through a dimmer switch. The load control device comprisesa load control circuit adapted to be coupled to the electrical load forcontrolling the amount of power delivered to the electrical load, and acontroller coupled to the load control circuit for controlling theamount of power delivered to the electrical load in response to aphase-control voltage received from the dimmer switch. The load controldevice further comprises a controllable-load circuit coupled to thecontroller and operable to conduct a controllable-load current from theAC power source through the dimmer switch. The controller is configuredto determine whether the phase-control voltage comprises a forwardphase-control waveform or a reverse phase-control waveform such that thecontroller is configured to operate in a first mode when thephase-control voltage is a forward phase-control waveform, and tooperate in a second mode when the phase-control voltage is a reversephase-control waveform. The controller is operable to enable thecontrollable-load circuit, such that the controllable-load circuitconducts the controllable-load current through the dimmer switch whenthe controller is operating in the first mode. The controller is furtheroperable to disable the controllable-load circuit, such that thecontrollable-load circuit does not conduct the controllable-load currentthrough the dimmer switch when the controller is operating in the secondmode. A magnitude of the controllable-load current conducted by thecontrollable-load circuit through the dimmer switch may exceed at leastone of a rated latching current or a rated holding current of the dimmerswitch.

According to a fifth embodiment of the present invention, an LED driverfor controlling the amount of power delivered from an AC power source toan LED light source comprises a bus capacitor, an LED drive circuit, anLED drive circuit, a controller, and a controllable-load circuitoperable to conduct a controllable-load current from the AC power sourcethrough a dimmer switch, which is adapted to be coupled between the ACpower source and the load control device. The LED drive circuit isadapted to be coupled to the electrical load for controlling the amountof power delivered to the LED light source. The controller is coupled tothe LED drive circuit for controlling the amount of power delivered tothe LED light source in response to a phase-control voltage receivedfrom the dimmer switch. The controller is operable to determine whetherthe phase-control voltage comprises a forward phase-control waveform ora reverse phase-control waveform. The controller is operable to enablethe controllable-load circuit, such that the controllable-load circuitconducts the controllable-load current through the dimmer switch whenthe phase-control voltage is a forward phase-control waveform. Thecontroller is further operable to disable the controllable-load circuit,such that the controllable-load circuit does not conduct thecontrollable-load current through the dimmer switch when thephase-control voltage is a reverse phase-control waveform.

In addition, a controllable-load circuit for drawing a selected currentfrom a power source (e.g., an alternating-current source) is describedherein. The controllable-load circuit may be coupled across the powersource that is providing power to a driver circuit for driving alight-emitting diode (LED) light source. The controllable-load circuitcomprises: (1) a first semiconductor switch having a control inputcoupled to the power source; (2) a first impedance-forming circuitconnected in series with the first semiconductor switch across the powersource; (3) a control circuit having an output coupled to a first inputof the first impedance-forming circuit for selecting between at leasttwo impedances to be connected in series with the first semiconductorswitch; and (4) a second circuit coupled to the first semiconductorswitch for monitoring the power source and for increasing the impedanceto current flowing from the power source through the first semiconductorswitch at a selected voltage level of the power source, thereby toreduce the current through the first semiconductor switch. The controlcircuit is configured to control the impedance of the impedance formingcircuit to automatically control the current through the firstsemiconductor switch so that it is at a predefined level. The controlcircuit is configured to control the impedance of the firstimpedance-forming circuit for each half-cycle of the alternating-currentsource.

Other features and advantages of the present invention will becomeapparent from the following description of the invention that refers tothe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in greater detail in the followingdetailed description with reference to the drawings in which:

FIG. 1A is a simplified block diagram of a load control system includinga digital dimmer switch and an LED driver for controlling the intensityof an LED light source according to a first embodiment of the presentinvention;

FIG. 1B is a simplified block diagram of an alternative load controlsystem including an analog dimmer switch and an LED driver forcontrolling the intensity of an LED light source according to the firstembodiment of the present invention;

FIG. 2A is an example waveform of a forward phase-control waveform;

FIG. 2B is an example waveform of a reverse phase-control waveform;

FIG. 3 is a simplified block diagram of the LED driver of FIGS. 1A and1B;

FIG. 4 is a simplified diagram of the controllable-load current drawn bythe controllable-load circuit of the LED driver of FIG. 3 when thereceived phase-control voltage is a forward phase-control waveform;

FIG. 5 is a simplified schematic diagram of the controllable-loadcircuit of the LED driver of FIG. 3;

FIG. 6 is a simplified flowchart of a startup procedure executed by acontrol circuit of the LED driver of FIG. 3 when power is first appliedto the LED driver;

FIG. 7 is a simplified flowchart of a phase control determinationprocedure executed by the control circuit of the LED driver of FIG. 3;

FIG. 8 is a simplified flowchart of a controllable load currentprocedure executed periodically by the control circuit of the LED driverof FIG. 3;

FIG. 9 is a simplified flowchart of a target light level measurementprocedure executed by the control circuit of the LED driver of FIG. 3 inresponse to changes of a target intensity;

FIG. 10 is a simplified flowchart of a controllable load currentadjustment procedure executed by the control circuit of the LED driverof FIG. 3 in response to changes of a target intensity;

FIG. 11 is a simplified block diagram of a load control system includinga dimmer switch and multiple LED drivers for controlling the intensityof respective LED light sources according to a second embodiment of thepresent invention;

FIG. 12 is a simplified flowchart of a controllable-load currentprocedure executed by a control circuit of the LED driver according to athird embodiment of the present invention;

FIG. 13 is a simplified flowchart of a controllable-load currentprocedure executed by a control circuit of the LED driver according to afourth embodiment of the present invention;

FIG. 14 is a simplified flowchart of a startup procedure executed by acontrol circuit of the LED driver according to a fifth embodiment of thepresent invention when power is first applied to the LED driver; and

FIG. 15 is a simplified flowchart of an LED driver discovery procedureexecuted by a control circuit of the LED driver according to the fifthembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The foregoing summary, as well as the following detailed description ofthe preferred embodiments, is better understood when read in conjunctionwith the appended drawings. For the purposes of illustrating theinvention, there is shown in the drawings an embodiment that ispresently preferred, in which like numerals represent similar partsthroughout the several views of the drawings, it being understood,however, that the invention is not limited to the specific methods andinstrumentalities disclosed.

FIG. 1A is a simplified block diagram of a load control system 100including a light-emitting diode (LED) driver 102 for controlling theintensity of an LED light source 104 (e.g., an LED light engine)according to a first embodiment of the present invention. The LED driver102 is coupled to an alternating-current (AC) power source 106 via adimmer switch 108 (e.g., a two-wire “digital” or “smart” dimmer switchas shown in FIG. 1A). The dimmer switch 108 as shown in FIG. 1A does notrequire a connection to the neutral side N of the AC power source 106.Alternatively, the dimmer switch 108 may comprise a three-wire dimmerswitch which would require a connection to the neutral side N of the ACpower source. The dimmer switch 108 generates a phase-control voltageV_(PC) (e.g., a dimmed-hot voltage), which may comprise a forwardphase-control waveform, i.e., a leading-edge phase-control voltage (asshown in FIG. 2A), or a reverse phase-control waveform, i.e., atrailing-edge phase-control voltage (as shown in FIG. 2B). The LEDdriver 102 is coupled to the dimmer switch 108 (for receipt of thephase-control voltage V_(PC)) and to the neutral side N of the AC powersource 106. The LED driver 102 is operable to control the intensity ofthe LED light source 104 in response to the phase-control voltageV_(PC). The dimmer switch 108 comprises a user interface 108A includingan intensity adjustment actuator (not shown), such as a slider controlor a rocker switch, for allowing a user to adjust the intensity of theLED light source 104.

The dimmer switch 108 comprises a bidirectional semiconductor switch108B, such as, for example, a thyristor (such as a triac) or twofield-effect transistors (FETs) coupled in anti-series connection, forgenerating the phase-controlled voltage V_(PC). Using a standardphase-control dimming technique, a dimmer switch control circuit 108Crenders the bidirectional semiconductor switch 108B conductive at aspecific time each half-cycle of the AC power source, such that thebidirectional semiconductor switch remains conductive for a conductionperiod T_(CON) during each half-cycle of the AC power source 106. TheLED driver 102 controls the amount of power delivered to the LED lightsource 104 in response to the phase-controlled voltage V_(PC) providedby the dimmer switch 108. The LED driver 102 is operable to turn the LEDlight source 104 on and off in response to the conductive period T_(CON)of the phase-control voltage V_(PC) received from the dimmer switch 108.In addition, the LED driver 102 is operable to adjust the intensity ofthe LED light source 104 to a target intensity L_(TRGT), which rangesbetween a low-end intensity L_(LE) (e.g., approximately 1%) and ahigh-end intensity L_(HE) (e.g., approximately 100%) in response to thephase-control voltage V_(PC). Specifically, the LED driver 102 controlsat least one of a load voltage V_(LOAD) across the LED light source 104or a load current I_(LOAD) through the LED light source to control theamount of power delivered to the LED light source.

The dimmer switch 108 also often comprises a power supply 108D coupledacross the bidirectional semiconductor switch 108B for powering thecontrol circuit 108C. The power supply 108D generates a DC supplyvoltage V_(PS) by drawing a charging current I_(CHRG) from the AC powersource 106 through the LED driver 102 when the bidirectionalsemiconductor switch 108B is non-conductive each half-cycle. Examples ofdigital dimmer switches having power supplies are described in greaterdetail in U.S. Pat. No. 5,248,919, issued Sep. 29, 1993, entitledLIGHTING CONTROL DEVICE, and in U.S. Pat. No. 6,969,959, issued Nov. 29,2005, entitled ELECTRONIC CONTROL SYSTEMS AND METHODS, the entiredisclosures of which are hereby incorporated by reference.

FIG. 1B is a simplified block diagram of an alternative load controlsystem 100′ comprising a two-wire “analog” dimmer switch 108′ forcontrolling the intensity of the LED light source 104 through the LEDdriver 102. The analog dimmer switch 108′ includes a timing circuit 108Eand a trigger circuit 108F rather than the dimmer control circuit 108Cand the power supply 108D of the digital dimmer switch 108 of FIG. 1A.As shown in FIG. 1B, the bidirectional semiconductor switch 108B of theanalog dimmer switch 108′ is implemented as a triac T1. The timingcircuit 108E is coupled in parallel electrical connection with the triacT1 and comprises, for example, a resistor R1 and a capacitor C1. Thetrigger circuit 108F is coupled between a gate of the triac T1 and thejunction of the resistor R1 and the capacitor C1, and comprises, forexample, a diac D1. The capacitor C1 of the timing circuit 108E chargesby conducting a timing current I_(TIM) from the AC power source 106 andthrough the resistor R1 and the LED driver 102 when the triac T1 isnon-conductive each half-cycle. When the voltage across the capacitor C1exceeds approximately a break-over voltage of the diac D1, the diacconducts current through the gate of the triac T1, thus, rendering thetriac conductive for the conduction period T_(CON). After the triac T1is fully conductive, the timing current I_(TIM) ceases to flow. As shownin FIG. 1B, the resistor R1 is a potentiometer having a resistanceadjustable in response to the user interface 108A to control how quicklythe capacitor C1 charges and thus the conduction period T_(CON) of thephase-controlled voltage V_(PC).

FIG. 3 is a simplified block diagram of the LED driver 102 according tothe first embodiment of the present invention. The LED driver 102comprises a radio-frequency interference (RFI) filter and rectifiercircuit 110, which receives the phase-control voltage V_(PC) from thedimmer switch 108. The RFI filter and rectifier circuit 110 operates tominimize the noise provided on the AC power source 106 and to generate arectified voltage V_(RECT). A buck-boost flyback converter 120 receivesthe rectified voltage V_(RECT) and generates a direct-current (DC) busvoltage V_(BUS) across a bus capacitor C_(BUS). The flyback converter120 may alternatively comprise any suitable power converter circuit forgenerating an appropriate bus voltage. The flyback converter 120 mayalso provide electrical isolation between the AC power source 106 andthe LED light source 104, and operate as a power factor correction (PFC)circuit to adjust the power factor of the LED driver 102 towards a powerfactor of one. The LED driver 102 comprises an LED drive circuit 130comprising a switching power supply such as a convertor oralternatively, a linear regulator, which receives the bus voltageV_(BUS) and controls the amount of power delivered to the LED lightsource 104 so as to control the intensity of the LED light source. Anexample of an LED drive circuit is described in greater detail in U.S.patent application Ser. No. 12/813,908, filed Jun. 11, 2010, entitledLOAD CONTROL DEVICE FOR A LIGHT-EMITTING DIODE LIGHT SOURCE, the entiredisclosure of which is hereby incorporated by reference.

The LED driver 102 further comprises a control circuit 140 forcontrolling the operation of the LED drive circuit 130. The controlcircuit 140 may comprise, for example, a microcontroller or any othersuitable processing device or controller, such as, for example, aprogrammable logic device (PLD), a microprocessor, or an applicationspecific integrated circuit (ASIC). A resistive divider comprising tworesistors R142, R144 is coupled across the output of the RFI filter andrectifier circuit 110 and generates a scaled phase-control voltagecontrol signal V_(PC-S), which is provided to the control circuit 140.The scaled phase-control voltage control signal V_(PC-S) isrepresentative of the magnitude of the phase-control voltage V_(PC)received from the dimmer switch 108. The control circuit 140 is operableto determine the conduction period T_(CON) of the phase-control voltageV_(PC) (and thus the target intensity L_(TRGT) of the LED light source104) from the scaled phase-control voltage control signal V_(PC-S).

The LED drive circuit 130 controls a peak magnitude I_(PK) of a loadcurrent I_(LOAD) conducted through the LED light source 104 in responseto a peak current control signal V_(IPK). The control circuit 140receives a load current feedback signal V_(ILOAD), which isrepresentative of the magnitude of the load current I_(LOAD) flowingthrough the LED light source 104. The control circuit 140 also receivesa LED voltage feedback signal V_(LED-NEG), which is representative ofthe magnitude of the voltage at the negative terminal of the LED lightsource 104.

The control circuit 140 is operable to control the LED drive circuit130, such that the LED driver 102 is adapted to work with a plurality ofdifferent LED light sources, which may be rated to operate usingdifferent load control techniques, different dimming techniques, anddifferent magnitudes of load current and voltage. Specifically, thecontrol circuit 140 is operable to control the LED drive circuit 130 tocontrol the amount of power delivered to the LED light source 104 usingthe two different modes of operation: a current load control mode (i.e.,for using a current load control technique) and a voltage load controlmode (i.e., for using a voltage load control technique). In addition,the control circuit 140 may also adjust the magnitude to which the LEDdrive circuit 130 will control the load current I_(LOAD) through the LEDlight source 104 in the current load control mode, or the magnitude towhich the LED driver 102 will control the load voltage V_(LOAD) acrossthe LED light source 104 in the voltage load control mode. Whenoperating in the current load control mode, the control circuit 140 isoperable to control the intensity of the LED light source 104 using twodifferent dimming techniques: a pulse-width modulation (PWM) dimmingtechnique and a constant current reduction (CCR) dimming technique. Whenoperating in the voltage load control mode, the control circuit 140 isonly operable to adjust the amount of power delivered to the LED lightsource 104 using the PWM dimming technique.

The LED driver 102 also comprises a power supply 150, which receives therectified voltage V_(RECT) and generates a direct-current (DC) supplyvoltage V_(CC) for powering the control circuit 140 and otherlow-voltage circuitry of the LED driver. The control circuit 140 is alsocoupled to a memory 160 for storing the operational characteristics ofthe LED driver 102 (e.g., the load control mode, the dimming mode, andthe magnitude of the rated load voltage or current). The LED driver 102may also comprise a communication circuit 170, which may be coupled to,for example, a wired communication link or a wireless communicationlink, such as a radio-frequency (RF) communication link or an infrared(IR) communication link. The control circuit 140 may be operable toupdate the target intensity L_(TRGT) of the LED light source 104 or theoperational characteristics stored in the memory 160 in response todigital messages received via the communication circuit 170.

The LED driver 102 further comprises a controllable-load circuit 180coupled to the output of the RFI filter and rectifier circuit 110 forreceipt of the rectified voltage V_(RECT). The controllable-load circuit180 is operable to draw a controllable-load current I_(CL) through thedimmer switch 108 to ensure that the current conducted through thethyristor of the dimmer switch exceeds the rated latching and holdingcurrents of the thyristor. The control circuit 140 provides acontrollable-load enable control signal V_(CL-EN) to thecontrollable-load circuit 180 for enabling and disabling the conductionof the controllable-load current I_(CL), through the dimmer switch 108.Specifically, the control circuit 140 is operable to enable thecontrollable-load circuit 180 to conduct the controllable-load currentI_(CL), when the phase-control voltage V_(PC) is a forward phase-controlwaveform, and to disable the controllable-load circuit 180 when thephase-control voltage V_(PC) is a reverse phase-control waveform. Inaddition, the control circuit 140 provides a plurality ofcontrollable-load adjustment control signals V_(CL-ADJ1), V_(CL-ADJ2),V_(CL-ADJ3), V_(CL-ADJ4) for adjusting the magnitude of thecontrollable-load current I_(CL) as will be described in greater detailbelow. In addition, the controllable-load circuit 180 may be operable toconduct the charging current I_(CHRG) of the power supply 108D of thedimmer switch 108 each half-cycle (i.e., the controllable-load circuitmay provide a path for the charging current of the power supply toflow).

FIG. 4 is a simplified diagram of the controllable-load current I_(CL)drawn by the controllable-load circuit 180 each half-cycle of the ACpower source 106 when the phase-control voltage V_(PC) is a forwardphase-control waveform received from the dimmer switch 108. After thethyristor of the dimmer switch 108 is rendered conductive (at thebeginning of a first period T₁ of FIG. 4), the voltage across the LEDdriver 102 and thus, the rectified voltage V_(RECT) quickly increase inmagnitude. During the first period T₁ of each half-cycle, thecontrollable-load circuit 180 conducts a pulse of current that has amagnitude dependent upon the instantaneous magnitude of the rectifiedvoltage V_(RECT), which exceeds the rated latching current of thethyristor of the dimmer switch 108 to ensure that the thyristor latches.During a second period T₂ of the half-cycle, the controllable-loadcircuit 180 operates as a constant current sink to draw a constantcurrent I_(CL-CNST), which has a magnitude that exceeds the ratingholding current of the thyristor to ensure that the thyristor remainsconductive. The magnitude of the constant current I_(CL-CNST) of thecontrollable-load current I_(CL), during the second period T₂ is lessthan the controllable-load current I_(CL), during the first period T₁and is not needlessly large resulting in unnecessary power dissipationin the controllable-load circuit 180 each half-cycle. The controlcircuit 140 is operable to control the magnitude of the constant currentI_(CL-CNST) of the controllable-load current I_(CL) during the secondperiod T₂ using the controllable-load adjustment control signalsV_(CL-ADJ1)-V_(CL-ADJ4). During a final, third period T₃ of eachhalf-cycle, the magnitude of the controllable-load current I_(CL) is notlimited to the constant current I_(CL-CNST) and decreases as themagnitude of the rectified voltage V_(RECT) decreases.

FIG. 5 is a simplified schematic diagram of the controllable-loadcircuit 180. As previously noted, the controllable-load circuit 180receives the rectified voltage V_(RECT) from the RFI filter andrectifier circuit 110. The controllable-load circuit 180 comprises asemiconductor switch, e.g., a field-effect transistor (FET) Q210, forconducting the controllable-load current I_(CL) when the FET Q210 isconductive. The drain of the FET Q210 is coupled to the rectifiedvoltage V_(RECT) through a resistor R212 (e.g., having a resistance ofapproximately 330Ω). The gate of the FET Q210 is coupled to anenergy-storage capacitor C214 (e.g., having a capacitance ofapproximately 0.044 μF) via two resistors R216, R218 (e.g., havingresistances of approximately 22 kΩ and 100 kΩ, respectively). The FETQ210 is rendered conductive when the voltage at the gate of the FETexceeds the rated gate voltage of the FET. The capacitor C214 chargesfrom the rectified voltage V_(RECT) through a diode D220 and a resistorR222 (e.g., having a resistance of approximately 100 kΩ), and operatesto maintain the appropriate gate voltage at the gate of the FET Q210independent of the instantaneous magnitude of the rectified voltageV_(RECT). A capacitor C224 (e.g., having a capacitance of approximately0.047 μF) and a zener diode Z225 are coupled between the gate and thesource of the FET Q210.

The controllable-load circuit 180 further comprises an NPN bipolarjunction transistor (BJT) Q226 coupled between the gate of the FET Q210and circuit common. The controllable-load enable control signalV_(CL-EN) is coupled to the base of the transistor Q226 via a resistorR228 (e.g., having a resistance of approximately 100 kΩ). When thecontrol circuit 140 drives the controllable-load enable control signalV_(CL-EN) high (i.e., to approximately the DC supply voltage V_(CC)),the transistor Q226 is rendered conductive, thus pulling the gate of theFET Q210 down towards circuit common. Accordingly, the PET Q210 isrendered non-conductive and the conduction of the controllable-loadcurrent I_(CL) is disabled. When the control circuit 140 drives thecontrollable-load enable control signal V_(CL-EN) low (i.e., toapproximately circuit common), the transistor Q226 is renderednon-conductive and the FET Q210 operates to conduct thecontrollable-load current I_(CL), (i.e., the conduction of thecontrollable-load current I_(CL), is enabled).

The source of the FET Q210 is coupled to circuit common via a pluralityof resistors R230, R232, R234, R236 and respective NPN bipolar junctiontransistors Q240, Q242, Q244, Q246 to create an impedance-formingcircuit. The bases of the transistors Q240, Q242, Q244, Q246 are coupledto respective resistors R250, R252, R254, R256, which each have aresistance, for example, of approximately 100 kΩ. The controllable-loadadjustment control signals V_(CL-ADJ1)-V_(CL-ADJ4) are coupled to thebases of the respective transistors Q240-Q246 via the respectiveresistors R250-R256. The control circuit 140 is operable to control thecontrollable-load adjustment control signals V_(CL-ADJ1)-V_(CL-ADJ4),for example, in a digital manner in order to provide fifteen differentmagnitudes of the constant current I_(CL-CNST) of the controllable-loadcurrent I_(CL) during the second period T₂ of each half-cycle as will bedescribed in greater detail below. For example, the first resistor R230may have a resistance of approximately 25Ω, the second resistor R232 mayhave a resistance of approximately 50Ω, the third resistor R234 may havea resistance of approximately 100Ω, and the fourth resistor R236 mayhave a resistance of approximately 200Ω. The control circuit 140controls at least one of the transistors Q240-Q246 to be conductive atall times, but may control more than one of the transistors to beconductive to couple two or more of the resistors R230-R236 in parallel.Accordingly, the impedance provided between the source of the FET Q210and circuit common may range between approximately a minimum resistanceR_(MIN) (e.g., approximately 13.33Ω) when all of the transistorsQ240-Q246 are conductive, and a maximum resistance R_(MAX) (e.g.,approximately 200Ω) when only the fourth transistor Q246 is conductive.

Two NPN bipolar junction transistors Q260, Q262 are coupled in seriesbetween the junction of the resistors R216, R218 and circuit common. Thebase of the first transistor Q260 is coupled to the source of the FETQ210 via a resistor R264 (e.g., having a resistance of approximately 1kΩ) and to circuit common via a resistor R266 (e.g., having a resistanceof approximately 1 kΩ). The first transistor Q260 operates to controlthe magnitude of the constant current I_(CL-CNST) of thecontrollable-load current I_(CL) conducted through the FET Q210 duringthe second period T₂ of each half-cycle as will be described in greaterdetail below.

The second transistor Q262 is controlled to be conductive andnon-conductive in response to the magnitude of the rectified voltageV_(RECT) to control when the first transistor Q260 limits the magnitudeof the constant current I_(CL-CNST) of the controllable-load currentI_(CL). A resistive divider comprising two resistors R268, R270 iscoupled across the output of the RFI filter and rectifier circuit 110and generates a scaled version of the rectified voltage V_(RECT). Acapacitor C272 is coupled in parallel with the resistor R270 and has acapacitance of, for example, approximately 47 pF. The base of thetransistor Q262 is coupled to the junction of the two resistors R268,R270 through a zener diode Z274 and to circuit common through a resistorR276 (e.g., having a resistance of approximately 100 kΩ). For example,the resistors R268, R270 of the resistive divider may have resistancesof approximately 2 MΩ and 392 kΩ, respectively, and the zener diode Z274may have a breakover voltage of approximately 5.6 V.

When the phase-control voltage V_(PC) received by the LED driver 102 isa forward phase-control waveform (as shown in FIG. 4), thecontrollable-load circuit 180 may not begin to conduct thecontrollable-load current I_(CL), at the beginning of each half-cycle ofthe AC power source 106 since the magnitude of the rectified voltageV_(RECT) may be approximately zero volts at this time and the capacitorC214 may not be charged to an appropriate voltage to render the FET Q210conductive. When the thyristor of the dimmer switch 108 is renderedconductive (at the beginning of the first period of time T₁), themagnitude of the rectified voltage V_(RECT) increases and the capacitorC214 charges quickly, such that the I-ET Q210 is driven into saturationand the controllable-load circuit 180 begins to conduct thecontrollable-load current I_(CL). The controllable-load circuit 180operates to conduct enough current through the thyristor of the dimmerswitch 108 to exceed the rated latching current of the thyristor duringthe first period of time T₁. At the beginning of the first period T₁,the capacitor C272 also begins to charge through the resistor R268. Thetransistor Q262 is rendered conductive when the magnitude of the voltageacross the capacitor C272 exceeds the rated base-emitter voltage of thetransistor for the duration of the second period of time T₂.Accordingly, the length of the first period T₁ is thus determined by theamount of time required to charge the capacitor C272 to the ratedbase-emitter voltage of the transistor Q262. Similarly, as the magnitudeof the rectified voltage V_(RECT) decreases, the magnitude of thevoltage across the capacitor C272 drops below the rated base-emittervoltage of the transistor Q262, thus rendering the transistornon-conductive at the beginning of the third period of time T₃ such thatthe FET Q210 is driven into saturation once again at the end of the halfcycle.

When the transistor Q262 is rendered conductive, the transistor Q260controls the magnitude of the controllable-load current I_(CL) to theconstant current I_(CL-CNST) during the second period of time T₂ inorder to conduct enough current through the thyristor of the dimmerswitch 108 to exceed the rated holding current of the thyristor. Duringthe second period of time T₂, the transistor Q260 operates in the linearregion and pulls the gate voltage of the FET Q210 down towards circuitcommon, thus causing the FET Q210 to also operate in the linear region.The magnitude of the constant current I_(CL-CNST) of thecontrollable-load current I_(CL) is dependent upon the impedanceprovided between the source of the FET Q210 and circuit common.Accordingly, the control circuit 140 is operable to selectively renderthe transistors Q240-Q246 conductive and non-conductive to adjust theimpedance provided between the source of the FET Q210 and circuitcommon, and thus to control the magnitude of the constant currentI_(CL-CNST) to one of the fifteen different discrete magnitudes.

The magnitude of the constant current I_(CL-CNST) of thecontrollable-load current I_(CL) is controlled to a maximum currentI_(CL-MAX) (e.g., approximately 50 mA) when the impedance providedbetween the source of the FET Q210 and circuit common is the minimumresistance R_(MIN) (i.e., when all of the transistors Q240-Q246 arerendered conductive and all of the resistors R230-R236 are coupled inparallel). The magnitude of the constant current I_(CL-CNST) iscontrolled to a minimum current I_(CL-MIN) (e.g., approximately 3.5 mA)when the impedance provided between the source of the FET Q210 andcircuit common is a maximum resistance (i.e., when only the transistorQ246 is rendered conductive and only the resistor R236 is coupledbetween the source of the FET Q210 and circuit common). The controlcircuit 140 controls at least one of the transistors Q240-Q246 to beconductive at all times, but may control more than one of thetransistors to be conductive to coupled two or more of the resistorsR230-R236 in parallel. The control circuit 140 is operable to turn offthe conduction of the controllable-load current I_(CL), by driving thecontrollable-load enable control signal V_(CL-EN) high and rendering thetransistor Q226 conductive and the FET Q210 non-conductive.

Immediately after starting up (i.e., when power is first applied to theLED driver 102) when the dimmer switch 108 is on or immediately afterthe dimmer switch is first turned on, the control circuit 140 isoperable to determine whether the phase-control voltage V_(PC) is aforward phase-control waveform or a reverse phase-control waveform.Specifically, the control circuit 140 monitors the magnitude of thephase-control voltage V_(PC) using the scaled phase-control voltagecontrol signal V_(PC-S) to determine whether the phase-control voltageV_(PC) is a forward phase-control waveform or a reverse phase-controlwaveform. If the phase-control voltage V_(PC) is a forward phase-controlwaveform, the magnitude of the phase-control voltage will increase veryquickly from approximately zero volts to a large magnitude at a specifictime each half-cycle depending upon the length of the conduction periodT_(CON). If the phase-control voltage V_(PC) is a reverse phase-controlwaveform, the magnitude of the phase-control voltage will increaseslowly from approximately zero volts at the beginning of eachhalf-cycle.

Therefore, after detecting a voltage transition of the phase-controlvoltage V_(PC) (i.e., when the phase-control voltage increases fromapproximately zero volts to be greater than zero volts), the controlcircuit 140 is operable to compare the magnitude of the phase-controlvoltage V_(PC) to a voltage threshold V_(TH) (e.g., approximately 38-40volts). If the magnitude of the phase-control voltage V_(PC) isconsistently larger than the voltage threshold V_(TH), the controlcircuit 140 concludes that the phase-control voltage is a forwardphase-control waveform and enables the conduction of thecontrollable-load current I_(CL). Otherwise, the control circuit 140concludes that the phase-control voltage is a reverse phase-controlwaveform and disables the conduction of the controllable-load currentI_(CL). Because reverse phase-control waveforms are typically generatedby dimmer switches having FETs and FETs are not limited by latching orholding currents, there is no need to enable the controllable-loadcircuit 180 when the phase-control voltage is a reverse phase-controlwaveform.

FIG. 6 is a simplified flowchart of a startup procedure 600 executed bythe control circuit 140 when power is first applied to the LED driver102 at step 602. The control circuit 140 first enables the controllableload circuit 180 via the controllable-load enable control signalV_(CL-EN) and controls the magnitude of the constant current I_(CL-CNST)of the controllable-load current I_(CL) to the maximum currentI_(CL-MAX) (in the event that the phase-control voltage V_(PC) is aforward phase-control waveform.) Then, the control circuit 140 executesa phase control determination procedure 300 to determine whether thephase-control voltage V_(PC) from the dimmer switch 108 is a forwardphase-control waveform or a reverse phase-control waveform as shown inFIG. 7 and described further below. Next, if the phase-control voltageV_(PC) is not a forward phase-control waveform (i.e., it is a reversephase control waveform as shown in FIG. 2B) at step 606, then thecontrol circuit 140 disables the controllable load circuit 180 via thecontrollable-load enable control signal V_(CL-EN), before the startupprocedure 600 exits. If the phase-control voltage V_(PC) is a forwardphase-control waveform as shown in FIG. 2A, then the controllable loadcircuit 180 remains enabled before the startup procedure 600 exits.

FIG. 7 is a simplified flowchart of the phase control determinationprocedure 300 executed by the control circuit 140 of the LED driver 102.The control circuit 140 first initializes a number of counters s, m, n,h to zero at step 312, and then waits for the next voltage transition atstep 314. Specifically, the control circuit 140 waits while themagnitude of the phase-control voltage V_(PC) (as determined from thescaled phase-control voltage control signal V_(PC-S)) is greater than anoise threshold V_(NOISE) (e.g., approximately 10 V) at step 314. Whenthe magnitude of the phase-control voltage V_(PC) becomes less than orequal to the noise threshold V_(NOISE) at step 314, the control circuit140 waits for the next sampling time at step 316 to sample the scaledphase-control voltage control signal V_(PC-S). For example, the controlcircuit 140 may sample the scaled phase-control voltage control signalV_(PC-S) using a sampling period T_(SAMPLE) of approximately 35 μsec. Atthe next sampling time at step 316, the control circuit 140 samples thescaled phase-control voltage control signal V_(PC-S) at step 318. Thecontrol circuit 140 continues to sample the scaled phase-control voltagecontrol signal V_(PC-S) at step 318 until the magnitude of thephase-control voltage V_(PC) becomes greater than the noise thresholdV_(NOISE) at step 320.

When the magnitude of the phase-control voltage V_(PC) rises above thenoise threshold V_(NOISE) at step 320, the control circuit 140 samplesthe scaled phase-control voltage control signal V_(PC-S) a predeterminedmaximum number S_(MAX) of times (e.g., 10 samples) each half cycle overa predetermined number H_(MAX) of consecutive half-cycles (e.g., 10half-cycles). If at least a first predetermined number M_(MAX) (e.g.,approximately 7) of the samples are above the second voltage thresholdV_(TH2) (i.e., approximately 38 V) for at least a second predeterminednumber N_(MAX) (e.g., approximately 7) of the half-cycles, the controlcircuit 140 concludes that the phase-control voltage V_(PC) generated bythe dimmer switch 108 is a forward phase-control waveform and enablesthe controllable-load circuit 180. Otherwise, the control circuit 140assumes that the phase-control voltage V_(PC) is a reverse phase-controlwaveform and disables the controllable-load circuit 180.

Referring back to FIG. 7, the control circuit 140 increments the counters by one at step 322 in order to keep track of the number of times thatthe scaled phase-control voltage control signal V_(PC-S) has beensampled. If the magnitude of the phase-control voltage V_(PC) is greaterthan or equal to the voltage threshold V_(TH) at step 324, the controlcircuit 140 increments the counter m by one at step 326 to keep track ofthe number of samples greater than the voltage threshold V_(TH). If themagnitude of the phase-control voltage V_(PC) is less than the voltagethreshold V_(TH) at step 324, the control circuit 140 does not incrementthe counter m. If the value of the counter s is not equal to (i.e., isless than) the predetermined maximum number S_(MAX) of samples (i.e.,10) at step 328, the control circuit 140 waits for the correct time tosample the scaled phase-control voltage control signal V_(PC-S) at step316 and once again samples the scaled phase-control voltage controlsignal V_(PC-S) at step 318 and determines if the magnitude of thephase-control voltage V_(PC) is greater than the voltage thresholdV_(TH) at step 324.

When the value of the counter s has increased to the predeterminedmaximum number S_(MAX) of samples at step 328, the control circuit 140increments the counter h by one at step 330 to keep track of the numberof consecutive half-cycles during which the control circuit has sampledthe scaled phase-control voltage control signal V_(PC-S). If the valueof the counter m (i.e., the number of samples exceeding the voltagethreshold V_(TH)) is greater than or equal to the value of thepredetermined number M_(MAX) at step 332, the control circuit 140increments the counter n by one at step 334. If the value of the counterh is not equal to (i.e., less than) the maximum number H_(MAX) (i.e.,10) of half-cycles at step 336, the control circuit 140 resets thecounter s to zero at step 338 and then waits for the beginning of thenext half-cycle at step 314.

If the counter h is equal to the maximum number H_(MAX) of half-cyclesat step 336, the control circuit 140 determines if the value of thecounter n (i.e., the number of half-cycles containing at least the firstpredetermined number M_(MAX) of samples exceeding the voltage thresholdV_(TH)) is greater than or equal to the second predetermined numberN_(MAX) at step 340. If so, the control circuit 140 determines that thephase-control voltage V_(PC) is a forward phase-control waveform andthus, begins to operate in a forward phase control mode at step 342.Specifically, when operating in forward phase control mode, the controlcircuit 140 drives the controllable-load enable control signal V_(CL-EN)low to enable the controllable-load circuit 180 at step 342, before thestartup procedure 300 exits. If the value of the counter n is less thanthe second predetermined number N_(MAX) at step 340, the control circuit140 determines that the phase-control voltage V_(PC) is a reversephase-control waveform and thus, begins to operate in a reverse phasecontrol mode at step 342. Specifically, when operating in reverse phasecontrol mode, the control circuit drives the controllable-load enablecontrol signal V_(CL-EN) high to disable the controllable-load circuit180 at step 344, before the startup procedure 300 exits.

After the control circuit 140 has determined that the phase-controlvoltage V_(PC) is a forward phase-control waveform and has enabled thecontrollable-load circuit 180, the control circuit is operable togradually reduce the magnitude of the constant current I_(CL-CNST) drawnby the controllable-load circuit during the second period T₂ of eachhalf-cycle until the control circuit determines the lowest current atwhich the dimmer switch 108 operates properly. The control circuit 140monitors the conduction period T_(CON) of the phase-control voltageV_(PC) (via the scaled phase-control voltage control signal V_(PC-S)) todetermine if changes in the constant current I_(CL-CNST) of thecontrollable-load current I_(CL) affect the operation of the dimmerswitch 108. Specifically, large variations in the conduction timeT_(CON) may indicate that the operation of the dimmer switch 108 hasbeen affected by the change in the constant current I_(CL-CNST) drawn bythe controllable-load circuit 180. If the control circuit 140 determinesthat the constant current I_(CL-CNST) of the controllable-load currentI_(CL) has been decreased too low, the control circuit increases themagnitude of the constant current I_(CL-CNST) back to the last suitablelevel and stops adjusting the magnitude of the constant currentI_(CL-CNST).

FIG. 8 is a simplified flowchart of a controllable-load currentprocedure 700 executed periodically (e.g., once each line cycle) by thecontrol circuit 140 of the LED driver 102. The control circuit 140executes the controllable-load current procedure 700 to monitor changesof the target intensity L_(TRGT) (i.e., changes of the conduction periodT_(CON) of the phase-control voltage V_(PC)) and to reduce the magnitudeof the constant current I_(CL-CNST) of the controllable-load currentI_(CL) to the lowest possible magnitude. The magnitude of the constantcurrent I_(CL-CNST) of the controllable-load current I_(CL) needed toensure that the thyristor remains conductive may change as a result of achange in the conduction period T_(CON) of the phase-control voltageV_(PC), thus, changes of the target intensity L_(TRGT) are closelymonitored such that the magnitude of the constant current I_(CL-CNST) ofthe controllable-load current I_(CL), can be updated accordingly. Thecontrollable-load current procedure 700 is also executed immediatelyafter the startup procedure 600 exits.

Referring to FIG. 8, if the phase-control voltage V_(PC) is a reversephase-control waveform at step 704, the controllable-load currentprocedure 700 simply exits. However, if the phase-control voltage V_(PC)is a forward phase-control waveform at step 704, the control circuit 140determines whether there has been a change in the target intensityL_(TRGT) at step 706. If there is no change, then the controllable-loadcurrent procedure 700 exits. Otherwise, the control circuit 140 controlsthe magnitude of the constant current I_(CL-CNST) of thecontrollable-load current I_(CL) to the maximum current I_(CL-MAX) atstep 708 (to ensure that the thyristor of the dimmer switch 108 remainsconductive as the target intensity L_(TRGT) changes).

Next, the control circuit 140 executes a target light level measurementprocedure 400A as shown in FIG. 9 and described in greater detail below.Once the target light measurement procedure 400A is completed, thecontrol circuit 140 considers the target intensity L_(TRGT) to bestabilized (i.e., the phase control signal V_(PC) is no longer changingin response to user adjustment at the dimmer switch 108). Then, thecontrol circuit 140 executes a controllable-load current adjustmentprocedure 400B as shown in FIG. 10 and described in greater detailbelow. The controllable-load current adjustment procedure 400B graduallyreduces the magnitude of the constant current I_(CL-CNST) of thecontrollable-load current I_(CL) to the lowest possible magnitude beforethe controllable-load current procedure 700 exits.

FIG. 9 is a simplified flowchart of the target light level measurementprocedure 400A. The control circuit 140 first initializes counters x, y,z to zero at step 414. Next, the control circuit 140 measures the lengthof the conduction period T_(CON) of the phase-control voltage V_(PC)(via the scaled phase-control voltage control signal V_(PC-S)) at step416. At step 418, the control circuit 140 sets a previous conductionperiod T_(CON-PREV) equal to the length of the conduction period T_(CON)determined at step 416.

The control circuit 140 then waits for a predetermined number X_(MAX) ofconsecutive stable half-cycles (i.e., half-cycles having the sameconduction period T_(CON)), before attempting to adjust the magnitude ofthe constant current I_(CL-CNST) of the controllable-load currentI_(CL). After the beginning of the next half-cycle at step 420, thecontrol circuit 140 measures the conduction period T_(CON) of the newhalf-cycle at step 422. If the conduction period T_(CON) (measured atstep 422) is approximately equal to the previous conduction periodT_(CON-PREV) at step 424 (i.e., within a predetermined tolerance), thecontrol circuit 140 increments the counter x at step 426 to keep trackof the number of consecutive half-cycles having the same conductionperiod T_(CON). If the value of the counter x is not equal to thepredetermined number X_(MAX) at step 428, the controllable-load currentprocedure 400 loops around to determine the conduction period T_(CON) ofthe next half-cycle at steps 420 and 422. When the conduction periodT_(CON) (measured at step 422) is not equal to the previous conductionperiod T_(CON-PREV) at step 424, the control circuit 140 sets the valueof the counter x equal to zero at step 430 and sets the previousconduction period T_(CON-PREV) equal to the length of the conductionperiod T_(CON) (as determined at step 422) at step 432, before thecontrollable-load current procedure 400 loops around to determine theconduction period T_(CON) of the next half-cycle at steps 420 and 422.When the value of the counter x has become equal to the predeterminednumber X_(MAX) at step 428, the control circuit 140 sets at step 434 asteady-state conduction period T_(CON-SS) equal to the value of theconduction period T_(CON) (as last measured at step 424) before exitingthe target light level measurement procedure 400A.

FIG. 10 is a simplified flowchart of the controllable-load currentadjustment procedure 400B which is executed by the control circuit 140to begin to scale back the magnitude of the constant current I_(CL-CNST)of the controllable-load current I_(CL). At step 440, the controlcircuit 140 decreases the magnitude of the constant current I_(CL-CNST)of the controllable-load current I_(CL), by controlling thecontrollable-load adjustment control signals V_(CL-ADJ1)-V_(CL-ADJ4) toappropriately render the transistors Q240-Q246 conductive andnon-conductive to control the constant current I_(CL-CNST) to the nextlowest of the fifteen possible discrete magnitudes of the constantcurrent I_(CL-CNST). After the beginning of the next half-cycle at step442, the control circuit 140 measures the value of the conduction periodT_(CON) of the new half-cycle at step 444 and calculates aconduction-period-adjustment value ΔT_(CON) at step 446, i.e.,

$\begin{matrix}{{\Delta\; T_{CON}} = {{\frac{T_{CON} - T_{{CON} - {SS}}}{T_{{CON} - {SS}}}}.}} & \left( {{Equation}\mspace{14mu} 1} \right)\end{matrix}$If the value of the conduction-period-adjustment value ΔT_(CON) is lessthan or equal to a maximum conduction-period-adjustment toleranceΔT_(CON-MAX) (e.g., approximately 3%) at step 448, the control circuit140 increments the counter y by one at step 450 to keep track of thenumber of half-cycles during which the conduction period T_(CON) iswithin the tolerance of the steady-state conduction period T_(CON-SS).If the conduction-period-adjustment value ΔT_(CON) is greater than themaximum conduction-period-adjustment tolerance ΔT_(CON-MAX) at step 448,the control circuit 140 increments the counter z by one at step 452 tokeep track of the number of half-cycles during which the conductionperiod T_(CON) is outside of the tolerance (i.e., 3%) of thesteady-state conduction period T_(CON-SS).

After incrementing the counter y at step 450 or the counter z at step452, the control circuit 140 determines if the value of the counter y isgreater than or equal to a maximum number Y_(MAX) of half-cycles (e.g.,approximately 20) at step 454 or if the value of the counter z isgreater than or equal to a maximum number Z_(MAX) of half-cycles (e.g.,approximately 12) at step 456. If the value of the counter y is lessthan the maximum number Y_(MAX) at step 454 and if the value of thecounter z is less than the maximum number Z_(MAX) at step 456, thecontrollable-load current adjustment procedure 400B loops around tomeasure the conduction period T_(CON) at step 444 and calculate theconduction-period-adjustment value ΔT_(CON) once again at step 446. Ifthe value of the counter y is greater than or equal to the maximumnumber Y_(MAX) of half-cycles at step 454 and the constant currentI_(CL-CNST) is not equal to the minimum current I_(CL-MIN) at step 458,the controllable-load current adjustment procedure 400B loops around toallow the control circuit 140 to decrease the magnitude of the constantcurrent I_(CL-CNST) of the controllable-load current I_(CL), once againat step 440. If the constant current I_(CL-CNST) is equal to the minimumcurrent I_(CL-MIN) at step 458, the controllable-load current adjustmentprocedure 400B simply exits. If the value of the counter z is greaterthan or equal to the maximum number Z_(MAX) of half-cycles at step 456(i.e., the control circuit 140 has determined that the constant currentI_(CL-CNST) of the controllable-load current I_(CL) has been adjustedtoo low), the control circuit increases the constant current I_(CL-CNST)back up to the next highest of the fifteen possible discrete magnitudesof the constant current I_(CL-CNST) at step 460 and thecontrollable-load current adjustment procedure 400B exits.Alternatively, after the control circuit increases the constant currentI_(CL-CNST) back up to the next highest of the fifteen possible discretemagnitudes of the constant current I_(CL-CNST) at step 460, thecontrollable-load current procedure 400B could include additionalverification to confirm that the constant current I_(CL-CNST) of thecontrollable-load current I_(CL) has been increased sufficiently (e.g.,in a manner similar to steps 442 through 456).

During the controllable-load current adjustment procedure 400B, thecontrol circuit 140 of the LED driver 102 is further operable tomaintain the light intensity of the LED light source 104 at a constantintensity while the constant current I_(CL-CNST) of thecontrollable-load current I_(CL), is adjusted such that anyuser-perceivable changes in the intensity are avoided. In other words,the length of the conduction period T_(CON) of the phase-control voltageV_(PC) (via the scaled phase-control voltage control signal V_(PC-S))may change in response to the reductions of the constant currentI_(CL-CNST) of the controllable-load current I_(CL), (as describedabove). However, the control circuit 140 is operable to disregard suchchanges in the conduction period T_(CON) during the controllable-loadcurrent adjustment procedure 400B in order to maintain the lightintensity of the LED light source 104 at a constant intensity.

FIG. 11 is a simplified block diagram of a load control system 500including the dimmer switch 108 and multiple LED drivers 102A, 102B,102C for controlling the intensity of respective LED light sources 104according to a second embodiment of the present invention. The LEDdrivers 102A, 102B, 102C are coupled in parallel and each operate in anidentical fashion as the LED driver 102 of the first embodiment so as tocontrol the intensity of the respective LED light sources 104 in unisonin response to the single phase-control voltage V_(PC) generated by thedimmer switch 108. The LED drivers 102A, 102B, 102C are each operable tocontrollably conduct the controllable-load current I_(CL) through thethyristor of the dimmer switch 108, such that the magnitude of the totalcurrent conducted through the thyristor of the dimmer switch 108 is thesum of the magnitudes of the respective controllable-load currentsI_(CL), of the LED drivers. The LED drivers 102A, 102B, 102C may each beoperable to conduct the respective controllable-load current I_(CL),when the phase-control voltage V_(PC) received from the dimmer switch108 is a forward phase-control waveform, and not to conduct thecontrollable-load current I_(CL) when the phase-control voltage V_(PC)is a reverse phase-control waveform. In addition, the LED drivers 102A,102B, 102C are each operable to reduce the magnitude of the constantcurrent I_(CL-CNST) of the respective controllable-load current I_(CL)independently of each other, such that the total magnitude of the totalcurrent conducted through the thyristor of the dimmer switch 108 can beaccordingly reduced per procedure 700 previously described with respectto the first embodiment.

According to a third embodiment of the present invention, the LEDdrivers 102A, 102B, and 102C may be operable to communicate with oneanother via the respective communication circuit 170 in order to reducethe total current conducted through the thyristor of the dimmer switch108 to the lowest magnitude. For example, the LED drivers 102A, 102B,102C may be operable to transmit and receive digital messages via acommunication link (not shown), such as a wired communication link or awireless communication link, for example, a radio-frequency (RF)communication link or an infrared (IR) communication link. Examples of aload control system having a communication link are described in greaterdetail in co-pending, commonly-assigned U.S. patent application Ser. No.11/644,652, filed Dec. 22, 2006, entitled METHOD OF COMMUNICATINGBETWEEN CONTROL DEVICES OF A LOAD CONTROL SYSTEM, and in U.S. Pat. No.5,905,442, issued May 18, 1999, entitled METHOD AND APPARATUS FORCONTROLLING AND DETERMINING THE STATUS OF ELECTRICAL DEVICES FROM REMOTELOCATIONS, the entire disclosures of which are hereby incorporated byreference.

According to the third embodiment, each LED driver 102A, 102B, 102C isoperable to independently reduce the magnitude of the constant currentI_(CL-CNST) of the respective controllable-load current I_(CL), usingthe target light level measurement procedure 400A (FIG. 9) and thecontrollable-load current adjustment procedure 400B (FIG. 10) of thefirst embodiment of the present invention. However, each LED driver102A, 102B, 102C is also operable to transmit and receive simple digitalmessages before each LED driver begins the controllable-load currentadjustment procedure 400B. Thus, each LED driver 102A, 102B, 102C cangradually reduce the magnitude of the constant current I_(CL-CNST) ofthe respective controllable-load current I_(CL) one at a time, such thatthe magnitude of the constant current is efficiently optimized for theload control system 500. According to the third embodiment of thepresent invention, the minimum amount of current I_(CL-MIN) may be zeroamps.

For example, in response to a change to the target intensity L_(TRGT),LED driver 102A may first gradually reduce the magnitude of the constantcurrent I_(CL-CNST) of the respective controllable-load current I_(CL)flowing through LED driver 102A. Since LED drivers 102B, 102C may alsobe conducting a respective controllable-load current I_(CL), LED driver102A may be operable to reduce the magnitude of the constant currentI_(CL-CNST) of its respective controllable-load current I_(CL) toapproximately zero amps. Then LED driver 102B may gradually reduce themagnitude of the constant current I_(CL-CNST) of the respectivecontrollable-load current I_(CL) flowing through LED driver 102B toapproximately zero amps since LED driver 102C may still be conductingits respective controllable-load current I_(CL). Finally, LED driver102C may begin to reduce the magnitude of the constant currentI_(CL-CNST) of its respective controllable-load current I_(CL). However,since the LED driver 102C is the only LED driver that is stillconducting the controllable-load current I_(CL), the LED driver 102C maynot be able to reduce the magnitude of the current to zero amps. Rather,the LED driver 102C would be operable to determine the minimum amount ofcontrollable-load current I_(CL) to be conducted through the thyristorof the dimmer switch 108 in a similar manner as the LED driver 102 ofthe first embodiment.

If each LED driver 102A, 102B, 102C were to simultaneously begin toreduce the magnitude of its respective controllable-load current I_(CL)in response to detecting a change in the target intensity L_(TRGT) (asmay occur according to the second embodiment), the magnitude of theconstant current may not be fully optimized. For example, if each LEDdriver 102A, 102B, 102C of the second embodiment reduces the magnitudeof the constant current I_(CL-CNST) of the respective controllable-loadcurrent I_(CL) too much (such that the thyristor no longer remainslatched), all three LED drivers 102A, 102B, 102C may then subsequentlyincrease the magnitude of the controllable-load current whereas only asingle LED driver may actually need to increase the magnitude of itscontrollable-load current. Thus, by ensuring that each LED driver 102A,102B, and 102C reduces the magnitude of the constant current I_(CL-CNST)of the respective controllable-load current I_(CL), one at a time (viathe transmission and reception of simple digital messages) according tothe third embodiment, the minimum amount of current needed to ensurethat the thyristor of the dimmer switch 108 remains latched can bereached.

FIG. 12 is a simplified flowchart of a controllable-load currentprocedure 800 executed by each respective control circuit 140 of the LEDdrivers 102A, 102B, 102C according to the third embodiment of thepresent invention. The controllable-load current procedure 800 of thethird embodiment is similar to the controllable-load current procedure700 of the first embodiment. If the phase-control voltage V_(PC) is areverse phase-control waveform at step 804, the controllable-loadcurrent procedure 800 simply exits. However, if the phase-controlvoltage V_(PC) is a forward phase-control waveform at step 804, theneach respective control circuit 140 of the LED drivers 102A, 102B, 102Cdetermines whether there is a change in the target intensity L_(TRGT) atstep 806. If there is no change, then the controllable-load currentprocedure 800 exits. Otherwise, each respective control circuit 140 ofthe LED drivers 102A, 102B, 102C controls the magnitude of the constantcurrent I_(CL-CNST) of the controllable-load current I_(CL) to themaximum current I_(CL-MAX) at step 808 (to ensure that the thyristor ofthe dimmer switch 108 remains conductive as the target intensityL_(TRGT) changes).

Next, each respective control circuit 140 of the LED drivers 102A, 102B,102C executes the target light level measurement procedure 400Apreviously described. Once the target light measurement procedure 400Ais completed, the target intensity L_(TRGT) is considered by eachcontrol circuit 140 to be stabilized (i.e., the phase control signalV_(PC) is no longer changing in response to user adjustment at thedimmer switch 108). Then, each respective control circuit 140 of the LEDdrivers 102A, 102B, 102C checks whether a start adjustment message hasbeen received via the communication link at step 812. The startadjustment message indicates whether one of the LED drivers 102A, 102B,102C has begun the controllable-load current adjustment procedure 400B.Step 812 may also include a randomized wait period (i.e., a randomback-off period) to prevent each respective control circuit 140 of theLED drivers 102A, 102B, 102C from simultaneously progressing to step 814described in greater below. The random back-off period would begenerated from a unique number, such as a serial number, stored in therespective memory 160 of each LED driver 102A, 102B, 102C.

If one of the control circuits, for example, the control circuit 140 ofthe LED driver 102A, has not received a start adjustment message fromthe other respective control circuits of LED drivers 102B, 102C at step812 (and if control circuit of the LED driver 102A has the shortestrandomized wait period), then the control circuit 140 of LED driver 102Atransmits the start adjustment message at step 814. Once the controlcircuit 140 of the LED driver 102A transmits the start adjustmentmessage, the respective control circuits 140 of the LED drivers 102B,102C receive the start adjustment message at step 812 shortlythereafter. The control circuits 140 of the LED drivers 102B, 102C thenwait to receive a stop adjustment message at step 816. The stopadjustment message indicates that one of the LED drivers 102A, 102B,102C has completed the controllable-load current adjustment procedure400B.

While the respective control circuits 140 of the LED drivers 102B, 102Care waiting at step 816, the control circuit 140 of the LED driver 102Aexecutes the controllable-load current adjustment procedure 400B toreduce the magnitude of the constant current I_(CL-CNST) of therespective controllable-load current I_(CL) through the LED driver 102Ato the lowest possible magnitude. Because the remaining control circuits140 of LED drivers 102B, 102C are still controlling the magnitude of theconstant current I_(CL-CNST) of the controllable-load current I_(CL) tothe maximum current I_(CL-MAX) (from step 808), the control circuit 140of the LED driver 102A may be able to reduce the magnitude of theconstant current I_(CL-CNST) of the controllable-load current I_(CL)through the LED driver 102A to the minimum current I_(CL-MIN). When thecontrollable-load current adjustment procedure 400B is complete, thecontrol circuit 140 of the LED driver 102A transmits the stop adjustmentmessage via the communication link at step 818 before the controlcircuit 140 exits the controllable-load current procedure 800.

When the control circuit 140 of the LED driver 102A transmits the stopadjustment message at step 818, the remaining respective controlcircuits 140 of the LED drivers 102B, 102C receive the stop adjustmentmessage at step 816. Then, the remaining control circuits 140 of the LEDdrivers 102B, 102C check whether another start adjustment message hasbeen received via the communication link at step 812. If a startadjustment message has not been received at step 812, one of the controlcircuits, for example, the control circuit 140 of LED driver 102B(having a shorter randomized wait period than the LED driver 102C),transmits the start adjustment message at step 814 before beginning thecontrollable-load current adjustment procedure 400B. Once the controlcircuit 140 of the LED driver 102C receives the start adjustment messagefrom the LED driver 102B, the control circuit 140 of the LED driver 102Cwaits at step 816 to receive the stop adjustment message.

The control circuit 140 of the LED driver 102B completes thecontrollable-load current adjustment procedure 400B to gradually reducethe magnitude of the constant current I_(CL-CNST) of thecontrollable-load current I_(CL) through the LED driver 102B to thelowest possible magnitude. Again because the remaining LED driver 102Cis still controlling the magnitude of the constant current I_(CL-CNST)of the controllable-load current I_(CL) to the maximum currentI_(CL-MAX) (from step 808), the control circuit 140 of the LED driver102B may be able to reduce the respective magnitude of the constantcurrent I_(CL-CNST) of the controllable-load current I_(CL) (through theLED driver 102B) to the minimum current I_(CL-MIN). After completing thecontrollable-load adjustment procedure 400B, the control circuit 140 ofthe LED driver 102B transmits the stop adjustment message at step 818before the control circuit 140 exits the controllable-load currentprocedure 800.

When the LED driver 102B transmits the stop adjustment message at step818, the remaining control circuit 140 of the LED driver 102C receivesthe stop adjustment message at step 816. Then, the control circuit 140of the LED driver 102C checks whether another start adjustment messagehas been received via the communication link at step 812. If no startadjustment message has been received at step 812, then finally, thecontrol circuit 140 of the LED driver 102C transmits the startadjustment message at step 814 before beginning the controllable-loadcurrent adjustment procedure 400B, transmits the stop adjustment messageat step 818, and exits controllable-load current procedure 800. Becausethe control circuit 140 of the LED driver 102C may be the only controlcircuit in the load control system 500 that is still controlling themagnitude of the constant current I_(CL-CNST) of the controllable-loadcurrent I_(CL), to a non-minimum current value, the control circuit maynot be able to completely reduce the magnitude of the constant currentI_(CL-CNST) of the controllable-load current I_(CL) through the LEDdriver 102C to the minimum current I_(CL-MIN). However, thecontrollable-load current adjustment procedure 400B provides that thecontrol circuit 140 of the LED driver 102C determines the lowestpossible magnitude of the constant current I_(CL-CNST) of thecontrollable-load current I_(CL) while still ensuring proper operationof the thyristor of the dimmer switch 108.

FIG. 13 is a simplified flowchart of a controllable-load currentprocedure 900 executed by a control circuit of the LED driver accordingto a fourth embodiment of the present invention. According to the fourthembodiment, the LED drivers 102A, 102B, and 102C may be operable tocommunicate with one another in a similar manner as described in thethird embodiment. However, the LED drivers 102A, 102B, 102C of thefourth embodiment operate such that if one of the LED drivers hasreduced the magnitude of the constant current I_(CL-CNST) of thecontrollable-load current I_(CL) to the minimum value I_(CL-MIN) (i.e.,zero amps), that one of the LED drivers will no longer communicate withthe other LED drivers or execute the controllable-load currentadjustment procedure 400B on a regular basis. For example, if the LEDdriver 102C becomes the only driver in the load control system 500 thatis still controlling the magnitude of the constant current I_(CL-CNST)of the controllable-load current I_(CL) to a non-minimum current value,LED driver 102C becomes the only LED driver that continues to executethe controllable-load current adjustment procedure 400B during thecontrollable-load current procedure 900. In other words, LED drivers102A and 102B may essentially ‘drop-out’ from the controllable-loadcurrent procedure 900 once they have reduced the magnitude of theconstant current I_(CL-CNST) of the controllable-load current I_(CL) tothe minimum value I_(CL-MIN) such that LED driver 102C becomes the onlydriver that continues to adjust the magnitude of the constant currentI_(CL-CNST) of the controllable-load current I_(CL) on a regular basisas further described below.

The controllable-load current procedure 900 of the fourth embodiment isvery similar to the controllable-load current procedure 800 of the thirdembodiment. The controllable-load current procedure 900 includes the useof a minimum controllable-load current I_(CL-MIN) flag to indicatewhether the LED drivers 102A, 102B, 102C are controlling the magnitudeof the constant current I_(CL-CNST) of the controllable-load currentI_(CL) to the minimum value. After determining that the phase-controlvoltage V_(PC) is a forward phase-control waveform at step 804, eachrespective control circuit 140 of the LED drivers 102A, 102B, 102C willcheck whether the minimum controllable-load current I_(CL-MIN) flag isset at step 902. When each respective control circuit 140 of the LEDdrivers 102A, 102B, 102C first executes the controllable-load currentprocedure 900, the minimum controllable-load current I_(CL-MIN) flag isnot set, thus each control circuit will execute steps 806-818 andprocedures 400A, 400B in a similar manner as described above withrespect to the controllable-load current procedure 800 of the thirdembodiment.

However, after each respective control circuit 140 of the LED drivers102A, 102B, 102C transmits the stop adjustment message at step 818, eachcontrol circuit determines whether the magnitude of the constant currentI_(CL-CNST) of the respective controllable-load current I_(CL) (as aresult of the controllable load current adjustment procedure 400B) isbeing controlled to the minimum value I_(CL-MIN) at step 904. If therespective control circuit 140 of LED drivers 102A, 102B, 102C iscontrolling the controllable current to the minimum value, then thecontrol circuit sets the minimum controllable-load current I_(CL-MIN)flag at step 906 before exiting the controllable-load current procedure900. Otherwise, each control circuit 140 of the LED drivers 102A, 102B,102C simply exits the controllable-load current procedure 900 withoutsetting the minimum controllable-load current I_(CL-MIN) flag.

Thus, if the LED driver 102C becomes the only driver that is stillcontrolling the magnitude of the constant current I_(CL-CNST) of thecontrollable-load current I_(CL) to a non-minimum current value, therespective control circuits 140 of LED drivers 102A and 102B will haveset the minimum controllable-load current I_(CL-MIN) flag, whereas thecontrol circuit of LED driver 102C will not have set the I_(CL-MIN)flag. As a result, the next time that the controllable-load currentprocedure 900 is executed by the control circuits 140 of the LED drivers102A, 102B, 102C, the control circuits of LED drivers 102A and 102B willdetermine that the minimum controllable-load current I_(CL-MIN) flag isset at step 902, and will immediately exit the procedure 900 (i.e.,“dropping out”). Further, since the control circuit 140 of the LEDdriver 102C has not set the minimum controllable-load current I_(CL-MIN)flag, LED driver 102C will continue to control the controllable loadcurrent according to steps 806-818, 904-906, and procedures 400A, B ofthe controllable-load current procedure 900.

Alternatively, the LED drivers 102A, 102B, 102C may not reduce themagnitude of the constant current I_(CL-CNST) in an incremental fashion(i.e., to one of the plurality of magnitudes between the maximum currentI_(CL-MAX) and approximately zero amps). The LED drivers 102A, 102B,102C may be operable to simply decrease (i.e., step) the magnitude ofthe constant current I_(CL-CNST) from the initial magnitude (i.e., themaximum current I_(CL-MAX)) to approximately zero amps. The LED drivers102A, 102B, 102C would each be operable to subsequently monitor thelength of the conduction period T_(CON) to determine if there have beenany changes in the length of the conduction period T_(CON), and toincrease the magnitude of the constant current I_(CL-CNST) back to themaximum current I_(CL-MAX) if the length of the conduction periodT_(CON) has changed. Accordingly, when operating together as part of thelighting control system, the LED drivers 102A, 102B, 102C wouldone-by-one decrease the magnitude of the constant current I_(CL-CNST) toapproximately zero amps (i.e., disable the respective controllable-loadcircuits 180) until the last LED driver determines that the magnitude ofthe constant current I_(CL-CNST) must remain at the maximum currentI_(CL-MAX) in order to conduct enough current through the thyristor ofthe dimmer switch 108 to exceed the rated latching and holding currentsof the thyristor.

According to a fifth embodiment of the present invention, the LEDdrivers 102A, 102B, 102C may be operable to determine a total numberN_(TOTAL) of LED drivers in the load control system 500 by transmittingand receiving digital messages via a discovery process that is executed,for example, upon power-up. Then, each LED driver 102A, 102B, 102C candetermine the appropriate magnitude of the constant current I_(CL-CNST)of the controllable-load current I_(CL) that each of the LED driversshould conduct, such that the magnitude of the total current conductedthrough the thyristor of the dimmer switch 108 will exceed the ratedlatching and holding currents of the thyristor. For example, each of theLED drivers 102A, 102B, 102C may control the magnitude of the respectiveconstant current I_(CL-CNST) as a function of the maximum currentI_(CL-MAX) and the total number N_(TOTAL) of LED drivers in the loadcontrol system 500, e.g.,I _(CL-CNST) =I _(CL-MAX) /N _(TOTAL).  (Equation 2)Accordingly, the magnitude of the total current conducted through thethyristor of the dimmer switch 108 is reduced to a low magnitude, andpower is not needlessly dissipated in each of the LED drivers 102A,102B, 102C.

FIG. 14 is a simplified flowchart of a startup procedure 1000, and FIG.15 is a simplified flowchart of an LED driver discovery procedure 1100executed by a control circuit 140 of the LED driver 102A, 102B, 102Caccording to the fifth embodiment. The startup procedure 1000 is verysimilar to the startup procedure 600 of the first embodiment. However,once each respective control circuit 140 has determined that thephase-control voltage V_(PC) is a forward phase-control waveform at step606, then each control circuit executes the LED driver discoveryprocedure 1100 to determine the total number N_(TOTAL) of LED drivers inthe load control system 500.

At step 1102 of the LED driver discovery procedure 1100 as shown in FIG.15, each respective control circuit 140 initializes the total numberN_(TOTAL) of LED drivers to one (i.e., each LED driver counts itself)and a query counter Q to zero. The query counter Q is used to count thenumber of query attempts, and at step 1104, each respective controlcircuit 140 determines whether the query counter Q is less than amaximum number of queries Q_(MAX) (e.g., 5). If the query counter Q isless than the maximum number of queries Q_(MAX), then the controlcircuit 140 transmits a query message (i.e., a “who's there?” message)via the communication link to determine whether there are other LEDdrivers in the load control system 500. After transmitting the querymessage, each respective control circuit 140 increments the querycounter at step 1108, and checks to see whether a response to the querymessage has been received at step 1110. The query message response willinclude a unique identifier of each LED driver 102A, 102B, 102C such asa serial number stored in each respective memory 160. If a query messageresponse has been received at step 1110, then the control circuit 140confirms whether the query message response contains a new serial numberat step 1112. If the serial number is new at step 1112, then the controlcircuit 140 will increment the total number N_(TOTAL) of LED drivers atstep 1114 before looping back to step 1104 to check the query counteragain. If the serial number is not new at step 1112, then the controlcircuit simply loops back to step 1104. In other words, each LED driver102A, 102B, 102C checks the serial number of the query message responseto ensure that the same LED driver is not counted more than once whendetermining the total number N_(TOTAL) of LED drivers.

If the control circuit 140 does not receive a query message response atstep 1110, then the control circuit determines whether it has received aquery message from another control circuit at step 1116. If the controlcircuit 140 has received a query message, then the control circuit sendsa query message response (including its respective serial number orunique identifier) at step 1118 before looping back to step 1104 tocheck the query counter again. Otherwise, the control circuit simplyloops back to step 1104. Once the query counter Q is no longer less thanthe maximum number of queries Q_(MAX), at step 1104, each respectivecontrol circuit 140 calculates the respective constant currentI_(CL-CNST) as a function of the maximum current I_(CL-MAX) and thetotal number N_(TOTAL) of LED drivers in the load control system 500 atstep 1120 before exiting procedure 1100. Thus, the fifth embodiment doesnot require each respective control circuit 140 of the LED drivers 102A,102B, 102C to regularly execute a controllable-load current procedure asdiscussed in the previous embodiments.

According to an alternate embodiment, the LED drivers 102A, 102B, 102Cmay be operable to communicate with a master control device via theirrespective communication circuits 170. The master control device may beoperable to send digital messages to each LED driver 102A, 102B, 102C toinstruct the LED drivers as to the appropriate magnitude of the constantcurrent I_(CL-CNST) of the respective controllable-load current I_(CL)that each LED driver should conduct. The master control device may beimplemented as a smart dimmer switch, a handheld programming device (forexample, a personal digital assistant or a smart cellular telephone), acentral controller (for example, a main repeater device for a wirelessload control system), or any type of suitable master control device.Alternatively, one of the LED drivers 102A, 102B, 102C may be operableto act as the master control device through an arbitration process as iswell known in the art. An example of such an arbitration process isdescribed in greater detail in co-pending, commonly-assigned U.S. patentapplication Ser. No. 11/672,884, filed Feb. 8, 2007, entitledCOMMUNICATION PROTOCOL FOR A LIGHTING CONTROL SYSTEM, the entiredisclosure of which is hereby incorporated by reference.

While the present invention has been described with reference to the LEDdrivers 102, 102A, 102B, 102C for controlling the intensity ofrespective LED light sources 104, the controllable-load circuit 180disclosed herein could be used in other load control devices, such aselectronic dimming ballasts for controlling fluorescent lamps, or motorcontrol devices for controlling motor loads. In addition, thecontrollable-load circuit 180 need not be implemented as part of theload control device (i.e., as part of the LED driver 102), but could beincluded in a separate enclosure and simply electrically coupled to thedimmer switch 108 of the load control system 100. Further, thecontrollable-load circuit 180 could alternatively be implemented as adigital potentiometer or a suitable equivalent.

In addition, while the phase control determination procedure 300disclosed herein is performed by the LED drivers 102, 102A, 102B, 102Cin order to appropriately control the controllable-load circuit 180, thephase control determination procedure could alternatively be performedto provide other capabilities. For example, the phase controldetermination procedure could be used to properly control the LED drivecircuit with an appropriate dimming range in response to determiningwhether the phase control signal is a forward or reverse phase controlwaveform. For example, once an LED driver has determined that thephase-control voltage V_(PC) is a forward phase-control waveform, theLED driver can operate in a forward phase control mode and use a forwardphase control dimming range when controlling the LED drive circuit.

Although the present invention has been described in relation toparticular embodiments thereof, many other variations and modificationsand other uses will become apparent to those skilled in the art. It ispreferred, therefore, that the present invention be limited not by thespecific disclosure herein, but only by the appended claims.

What is claimed is:
 1. A load control device for controlling the amountof power delivered from an AC power source to an electrical load, theload control device adapted to be coupled to the AC power source througha dimmer switch, the load control device comprising: a load controlcircuit adapted to be coupled to the electrical load for controlling theamount of power delivered to the electrical load; a controller coupledto the load control circuit for controlling the amount of powerdelivered to the electrical load in response to a conduction period of aphase-control voltage received from the dimmer switch; and acontrollable-load circuit coupled to the controller, thecontrollable-load circuit configured to conduct a controllable-loadcurrent from the AC power source through the dimmer switch, thecontrollable-load circuit configured to maintain the magnitude of thecontrollable-load circuit constant for at least a portion of eachhalf-cycle of the AC power source; wherein the controller is configuredto monitor the conduction period of the phase-control voltage and tocause the controllable-load circuit to decrease the magnitude of thecontrollable-load current from an initial magnitude in a firsthalf-cycle to a decreased magnitude in a second subsequent half-cycle,such that the conduction period of the phase-control voltage when thecontrollable-load current has the decreased magnitude in the secondhalf-cycle is the same as when the controllable-load current has theinitial magnitude in the first half-cycle.
 2. The load control device ofclaim 1, wherein the decreased magnitude is less than the initialmagnitude and greater than approximately zero amps.
 3. The load controldevice of claim 2, wherein the controller is configured to monitor theconduction period of the phase-control voltage after the magnitude ofthe controllable-load current has been decreased to the decreasedmagnitude in the second half-cycle and to further decrease the magnitudeof the controllable-load current by an increment in a third half-cycleif the conduction period of the phase-control voltage has not changed.4. The load control device of claim 3, wherein the controller isconfigured to increase the magnitude of the controllable-load current bythe increment in a fourth half-cycle if the conduction period of thephase-control voltage has changed.
 5. The load control device of claim1, wherein the decreased magnitude is approximately zero amps.
 6. Theload control device of claim 5, wherein the controller is configured tomonitor the conduction period of the phase-control voltage after themagnitude of the controllable-load current has been decreased toapproximately zero amps and to increase the magnitude of thecontrollable-load current in a subsequent half-cycle back to the initialmagnitude if the conduction period of the phase-control voltage haschanged.
 7. The load control device of claim 1, further comprising acommunications circuit coupled to the controller.
 8. The load controldevice of claim 7, wherein the controller is further configured todetermine a total number of load control devices coupled to the dimmerswitch and receiving the phase-control voltage.
 9. The load controldevice of claim 8, wherein the controller is further configured totransmit a query message and to receive a query response message via thecommunications circuit to determine the total number of load controldevices coupled to the dimmer switch.
 10. The load control device ofclaim 9, wherein the decreased magnitude of the controllable-loadcurrent is equal to a maximum magnitude divided by the total number ofload control devices coupled to the dimmer switch.
 11. The load controldevice of claim 7, wherein the controller is configured to transmit afirst digital message before decreasing the magnitude of thecontrollable-load current and to transmit a second digital message afterdecreasing the magnitude of the controllable-load current.
 12. The loadcontrol device of claim 1, wherein when the controller is configured tocause the controllable-load circuit to decrease the magnitude of thecontrollable-load current from the initial magnitude to the decreasedmagnitude, the controller further configured to control the load controlcircuit to deliver a constant amount of power to the electrical load.13. A load control system for controlling the amount of power deliveredfrom an AC power source to an electrical load, the load control systemcomprising: a load control device adapted to be coupled to theelectrical load for controlling the amount of power delivered to theelectrical load; and a dimmer switch comprising a thyristor coupled inseries electrical connection between the AC power source and the loadcontrol device, the thyristor characterized by a rated holding currentand configured to conduct a load current from the AC power source to theload control device, the dimmer switch configured to generate aphase-control voltage that is provided to the load control device toenable the load control device to adjust the amount of power beingdelivered to the electrical load in response to a conduction period ofthe phase-control voltage; wherein the load control device is configuredto conduct a controllable-load current from the AC power source throughthe thyristor of the dimmer switch, the load control device configuredto maintain the magnitude of the controllable-load current constant forat least a portion of each half-cycle of the AC power source, the loadcontrol device configured to decrease the magnitude of thecontrollable-load current from an initial magnitude in a firsthalf-cycle to a decreased magnitude in a second subsequent half-cycle,such that the decreased magnitude of the load current conducted throughthe thyristor in the second half-cycle exceeds the rated holdingcurrent.
 14. A load control system for controlling the amount of powerdelivered from an AC power source to first and second electrical loads,the load control system comprising: a first load control device adaptedto be coupled to the first electrical load for controlling the amount ofpower delivered to the first electrical load; a second load controldevice adapted to be coupled to the second electrical load forcontrolling the amount of power delivered to the second electrical load;and a dimmer switch comprising a thyristor coupled in series electricalconnection between the AC power source and the first and second loadcontrol devices, the thyristor characterized by a rated holding currentand configured to conduct a load current from the AC power source to thefirst and second load control devices, the dimmer switch configured togenerate a phase-control voltage that is provided to the first andsecond load control device to enable the first and second load controldevices to adjust the amount of power being delivered to the respectiveelectrical loads in response to a conduction period of the phase-controlvoltage; wherein the load control devices are configured to conductrespective controllable-load currents from the AC power source throughthe thyristor of the dimmer switch, the load control devices configuredto maintain the magnitude of each controllable-load current constant forat least a portion of each half-cycle of the AC power source, the loadcontrol devices each configured to adjust the magnitude of therespective controllable-load current from an initial magnitude in afirst half-cycle to a decreased magnitude in a second subsequenthalf-cycle, such that a total magnitude of the respectivecontrollable-load currents exceeds the rated holding current when thecontrollable-load currents have the respective decreased magnitudes inthe second half-cycle.
 15. The load control system of claim 14, whereinthe first load control device is configured to adjust the magnitude ofits respective controllable-load current from the initial magnitude to aminimum magnitude; and the second load control device is configured toadjust the magnitude of its respective controllable-load current from aninitial magnitude to a first magnitude greater than the minimummagnitude, such that the total magnitude of the respectivecontrollable-load currents exceeds the rated holding current of thethryristor.
 16. The load control system of claim 15, wherein the minimummagnitude is approximately zero amps.
 17. The load control system ofclaim 14, wherein the first and second load control devices are coupledto a communication link.
 18. The load control system of claim 17,wherein the first and second load control devices are configured todetermine a total number of load control devices in the load controlsystem.
 19. The load control system of claim 18, wherein the decreasedmagnitude of the respective controllable-load current is equal to amaximum magnitude divided by the total number of load control devices.20. The load control system of claim 19, wherein the initial magnitudeof the respective controllable-load current is equal to the maximummagnitude.
 21. The load control system of claim 17, wherein the firstload control device is configured to transmit a first digital messagebefore adjusting the magnitude of its respective controllable-loadcurrent from the initial magnitude to the decreased magnitude and asecond digital message after adjusting the magnitude of its respectivecontrollable-load current from the initial magnitude to the decreasedmagnitude.
 22. The load control system of claim 21, wherein the secondload control device is configured to adjust the magnitude of itsrespective controllable-load current from the initial magnitude to thedecreased magnitude in response to receiving the second digital messagefrom the first load control device.
 23. A controllable-load circuit fordrawing a selected current from a power source comprising: a firstsemiconductor switch having a control input coupled to the power source;a first impedance forming circuit connected in series with the firstsemiconductor switch across the power source, the first impedanceforming circuit including a first input for selecting between at leasttwo impedances to be connected in series with the first semiconductorswitch; a control circuit having an output coupled to said first inputfor selecting the selected impedance; and a second circuit coupled tosaid first semiconductor switch for monitoring the power source and forincreasing the impedance to current flowing from said power sourcethrough said first semiconductor switch at a selected voltage level ofsaid power source, thereby to reduce the current through said firstsemiconductor switch; wherein said power source comprises an alternatingcurrent source, said control circuit configured to control the impedanceof said impedance forming circuit to automatically control the currentthrough said first semiconductor switch to be at a predefined level,said control circuit configured to control the impedance of said firstimpedance forming circuit for each half-cycle of the alternating currentsource; further wherein the controllable-load circuit is coupled acrossthe power source providing power to a driver circuit driving an LEDlight source.
 24. The controllable-load circuit of claim 23, wherein thepower source comprises a source of rectified alternating current. 25.The controllable-load circuit of claim 24, wherein the rectifiedalternating current has a phase control component.
 26. Thecontrollable-load circuit of claim 25, wherein the phase controlcomponent comprises one of a forward phase control or a reverse phasecontrol.
 27. The controllable-load circuit of claim 26, wherein saidcontrol circuit is operable to determine whether said rectifiedalternating current comprises a forward phase control component or areverse phase control component.
 28. The controllable-load circuit ofclaim 27, wherein the control circuit monitors said power source todetermine if a voltage of said power source is above a predefinedvoltage level for a predefined time to determine if said power sourceincludes a forward phase component or a reverse phone component.
 29. Thecontrollable-load circuit of claim 25, wherein the control circuitcontrols the impedance of said first impedance forming circuit to setthe current through said first semiconductor switch at the predefinedlevel by determining if a conduction period of the phase controlcomponent is within a predefined value.
 30. The controllable-loadcircuit of claim 29, wherein said control circuit determines a steadystate value of the conduction period by determining if the conductionperiod remains constant for a predetermined number of half cycles ofsaid rectified alternating current, and if so, sets the determinedconduction period as the steady state conduction period.
 31. Thecontrollable-load circuit of claim 30, wherein to automatically controlthe current through the first semiconductor switch to the predefinedlevel, the control circuit implements the steps of: a) decreasing thecurrent through the first semiconductor switch by controlling the firstimpedance forming circuit to incrementally increase the impedance ofsaid impedance forming circuit; b) determining the present conductionperiod in the next half cycle of said rectified alternating current; c)determining a conduction period adjustment value as a function of saidsteady state conduction period and the present conduction period; d)determining if the conduction period adjustment value is less than orequal to a predefined conduction period adjustment value threshold; e)incrementing a within tolerance counter if the conduction periodadjustment value is less than the predefined conduction periodadjustment value threshold and incrementing an outside tolerance counterif the conduction period adjustment value is greater than the predefinedconduction period adjustment value threshold; f) determining if thewithin tolerance counter equals or exceeds a within tolerance maximumvalue and if so checking to determine if the current through the firstsemiconductor switch equals a predefined minimum value and if not,decreasing the current through the first semiconductor switch bycontrolling the first impedance forming circuit to incrementallyincrease the impedance of said impedance forming circuit and repeatingsteps b-f; and g) if the within tolerance counter is less than thewithin tolerance maximum value, determining if the outside tolerancecounter equals or exceeds a maximum value for the outside tolerancecounter and if not, repeating steps b-f, and if equal to the maximumvalue, controlling the first impedance forming circuit to incrementallydecrease the impedance of said impedance forming circuit toincrementally increase the current through said first semiconductorswitch.
 32. The controllable-load circuit of claim 31, wherein theconduction period adjustment value comprises a value expressed as theabsolute value of the difference between the determined presentconduction period and the steady state conduction period as a percentageof the steady state conduction period.
 33. The controllable-load circuitof claim 25, wherein the phase control component is generated by a phasecontrol circuit including a thyristor series connected with the firstsemiconductor switch, and wherein the predefined level of the currentthrough the first semiconductor switch minimally exceeds both a latchingcurrent and a holding current of said thyristor.
 34. Thecontrollable-load circuit of claim 23, wherein the second circuitcomprises a first transistor coupled to the control input of said firstsemiconductor switch and a second transistor in series with the firsttransistor, the second transistor having a control input coupled toreceive a voltage from said power source and to turn on at the selectedvoltage level to thereby cause current to be diverted away from thecontrol input of said first semiconductor switch to cause said firstsemiconductor switch to enable a linear region of operation.
 35. Thecontrollable-load circuit of claim 23, wherein the power sourcecomprises the output of a load control device including a seriesthyristor having a latching current and holding current, and saidcontrol circuit automatically adjusting said impedance of said impedanceforming circuit to a value that insures said predefined level of thecurrent through the first semiconductor switch is minimally adequate tomaintain the latching current and holding current through saidthyristor.
 36. The controllable-load circuit of claim 23, wherein thefirst impedance forming circuit comprises a plurality of parallelconnected series circuits each comprising a resistance and a controlledsemiconductor switch, each controlled semiconductor switch having aninput for selection by said control circuit, whereby one or moreimpedances can be disposed in series with said first semiconductorswitch to generate a plurality of incrementally different current valuesthrough said first semiconductor switch.
 37. A controllable-load circuitfor drawing a selected current from a power source comprising: a firstsemiconductor switch having a control input coupled to the power source;a first impedance forming circuit connected in series with the firstsemiconductor switch across the power source, the first impedanceforming circuit including a first input for selecting between at leasttwo impedances to be connected in series with the first semiconductorswitch; a control circuit having an output coupled to said first inputfor selecting the selected impedance, said control circuit configured tocontrol the impedance of said impedance forming circuit to automaticallycontrol the current through said first semiconductor switch to be at apredefined level; a second circuit coupled to said first semiconductorswitch for monitoring the power source and for increasing the impedanceto current flowing from said power source through said firstsemiconductor switch at a selected voltage level of said power source,thereby to reduce the current through said first semiconductor switch;wherein the power source comprises a source of rectified alternatingcurrent having a phase control component, the phase control componentcomprising one of a forward phase control component or a reverse phasecontrol component, said control circuit configured to determine whethersaid rectified alternating current comprises a forward phase controlcomponent or a reverse phase control component, said control circuitfurther configured to render the first semiconductor switch conductivewhen there is a forward phase control component and to render the firstsemiconductor switch nonconductive when there is a reverse phase controlcomponent.
 38. The controllable-load circuit of claim 37, wherein saidcontrol circuit is configured to sample the voltage of the power sourceand increment a first counter each time the voltage of the power sourceexceeds a noise level threshold to keep track of a number of samples.39. The controllable-load circuit of claim 38, wherein, if the voltageof the power source exceeds a noise level superimposed thereon, thecontrol circuit increments a second counter each time the voltage of thepower source exceeds a predefined voltage level and checks to determineif the number of samples equals a predefined number of samples.
 40. Thecontrollable-load circuit of claim 39, wherein the control circuitincrements a third counter each time the number of samples equals thepredefined number of samples to keep track of the number of half cyclesof said rectified alternating current and checks said second counter todetermine if said second counter equals or exceeds a predefined countvalue of said second counter.
 41. The controllable-load circuit of claim40, wherein, if the count of said second counter does not equal orexceed said second counter predefined count value, the third counter ischecked to determine if the third counter count equals a predefinedthird counter count and if not resets said first counter and furtherwherein if said second counter count equals or exceeds said secondcounter predefined count value, a fourth counter is incremented, saidthird counter count is checked to determine if it equals said predefinedthird counter count value and, if not said first counter is reset and ifit equals said predefined third counter count value, checks the fourthcounter count value to determine if it equals or exceeds a predefinedfourth counter count value, and if so, said control circuit determinesthat the phase control component is a forward phase control componentand enables said controllable-load circuit and if not, determines thatsaid phase control component is a reverse phase control component anddisables said controllable-load circuit.
 42. A load control device forcontrolling the amount of power delivered from an AC power source to anelectrical load, the load control device adapted to be coupled to the ACpower source through a dimmer switch, the load control devicecomprising: a load control circuit adapted to be coupled to theelectrical load for controlling the amount of power delivered to theelectrical load; a controller coupled to the load control circuit forcontrolling the amount of power delivered to the electrical load inresponse to a phase-control voltage received from the dimmer switch; anda controllable-load circuit coupled to the controller, thecontrollable-load circuit operable to conduct a controllable-loadcurrent from the AC power source through the dimmer switch; wherein thecontroller is operable to determine whether the phase-control voltagecomprises a forward phase-control waveform or a reverse phase-controlwaveform, the controller operable to operate in a first mode when thephase-control voltage is a forward phase-control waveform, and tooperate in a second mode when the phase-control voltage is a reversephase-control waveform; wherein the controller is operable to enable thecontrollable-load circuit, such that the controllable-load circuitconducts the controllable load current through the dimmer switch whenthe controller is operating in the first mode, the controller furtheroperable to disable the controllable-load circuit, such that thecontrollable-load circuit does not conduct the controllable-load currentthrough the dimmer switch when the controller is operating in the secondmode; and wherein a magnitude of the controllable load current conductedby the controllable-load circuit through the dimmer switch exceeds atleast one of a rated latching current or a rated holding current of thedimmer switch.
 43. The load control device of claim 42, wherein thecontroller monitors a magnitude of the phase-control voltage receivedfrom the dimmer switch to determine whether the phase-control voltage isa forward phase-control waveform or a reverse phase-control waveform.44. The load control device of claim 43, wherein the control circuitcompares the magnitude of the phase-control voltage to a voltagethreshold after detecting a voltage transition of the phase-controlvoltage to determine whether the phase-control voltage is a forwardphase-control waveform or a reverse phase-control waveform, thecontroller determining that the phase-control voltage is a forwardphase-control waveform if the magnitude of the phase-control voltage isconsistently larger than the voltage threshold.
 45. An LED driver forcontrolling the amount of power delivered from an AC power source to anLED light source, the LED driver adapted to be coupled to the AC powersource through a dimmer switch, the LED driver comprising: a buscapacitor for storing a DC bus voltage; an LED drive circuit coupled tothe bus capacitor for receiving the DC bus voltage and adapted to becoupled to the LED light source for controlling the amount of powerdelivered to the LED light source; a controller coupled to the LED drivecircuit for controlling the amount of power delivered to the LED lightsource to control the intensity of the LED light source in response to aphase-control voltage received from the dimmer switch; and acontrollable-load circuit coupled to the controller, thecontrollable-load circuit operable to conduct a controllable-loadcurrent from the AC power source through the dimmer switch; wherein thecontroller is operable to determine whether the phase-control voltagecomprises a forward phase-control waveform or a reverse phase-controlwaveform, the controller operable to enable the controllable-loadcircuit, such that the controllable-load circuit conducts thecontrollable-load current through the dimmer switch when thephase-control voltage is a forward phase-control waveform, thecontroller further operable to disable the controllable-load circuit,such that the controllable-load circuit does not conduct thecontrollable-load current through the dimmer switch when thephase-control voltage is a reverse phase-control waveform.
 46. The LEDdriver of claim 45, wherein the controller is further operable todecrease a magnitude of the controllable-load current from an initialmagnitude to a decreased magnitude when the controllable-load circuit isenabled.
 47. The LED driver of claim 46, wherein the decreased magnitudeof the controllable-load current conducted by the controllable-loadcircuit through the dimmer switch exceeds at least one of a ratedlatching current or a rated holding current of the dimmer switch. 48.The LED driver of claim 47, wherein the controller is operable tomonitor a conduction period of the phase-control voltage after themagnitude of the controllable-load current has been decreased to thedecreased magnitude and to further decrease the magnitude of thecontrollable-load current by an increment if the conduction period ofthe phase-control voltage has not changed.
 49. The LED driver of claim48, wherein the controller is operable to increase the magnitude of thecontrollable-load current by the increment if the conduction period ofthe phase-control voltage has changed.
 50. The LED driver of claim 46,wherein when the controller decreases the magnitude of thecontrollable-load current from the initial magnitude to the decreasedmagnitude, the controller is further operable to control the LED drivecircuit to deliver a constant amount of power to the LED light source.